Production of hollow ceramic membranes by electrophoretic deposition

ABSTRACT

The present invention provides methods for producing hollow ceramic membranes by electrophoretic deposition. The hollow ceramic membranes may have a small cross-sectional area of about 1.0×10 −5  mm 2  to about 25 mm 2 . The cross-sectional configuration of the hollow ceramic membranes may be any geometry such as circular, square, rectangular, triangular or polygonal. The hollow ceramic membranes produced by the methods of the present invention may have multiple layers but always the innermost layer, or the first deposited layer is porous and made by electrophoretic deposition. Subsequent layers may be porous or non porous and deposited before or after sintering the first layer. If it is deposited after sintering, it may require additional sintering steps. Additional layers may be deposited by further electrophoretic deposition, sol-gel coating, dip coating, vacuum casting, brushing, spraying or other known techniques.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/583,274 filed on May 31, 2000 which claimed thepriority benefit of Canadian Application No. 2,308,092 filed on May 10,2000.

BACKGROUND OF INVENTION

[0002] The present invention relates to the production of hollow ceramicmembranes by electrophoretic deposition. In particular, the presentinvention relates to the production of small cross-sectional area hollowceramic membranes by electrophoretic deposition.

[0003] It is well known to deposit coatings of material byelectrophoretic deposition (“EPD”). EPD is a combination ofelectrophoresis and deposition. Electrophoresis is the movement ofcharged particles in an electric field. Deposition is the coagulation ofparticles into a mass.

[0004] In U.S. Pat. No. 5,580,835 to Dalzell et al., a process forcreating ceramic fibers by EPD is described. The ceramic fibers producedby this process are fully dense, non-porous fibers. The described EPDprocess uses a colloidal metal hydrate from an aqueous sol where themetal hydroxide particle size is in the range of about 15 nm. The solsare produced by hydrolysis and peptization of an organometallic compoundin an aqueous medium. The resulting ceramic fiber is non-porous andfully dense as a result of the small particle size of the sol and thesintering process. Because the sol is aqueous, hydrogen evolution isunavoidable and steps must be taken to minimize hydrogen evolution andto permit hydrogen to escape such that it does not embed in thedeposited material. One means of doing so disclosed in this patent is touse a low potential and to continuously move the fiber during thedeposition process.

[0005] As is apparent in the Dalzell et al. Patent, it is conventionallybelieved that in order to achieve uniform deposition, only ceramicparticles of submicron size may be used in an EPD process. As a result,the resulting ceramic materials, after sintering, are not porous.

[0006] It is desirable for certain applications to produce a poroushollow ceramic fibre or membrane. Such fibres may be produced byextruding a mixture of ceramic powder and polymeric binder as disclosedin U.S. Pat. No. 5,707,584. The extruded tube or fibre may then be heattreated to remove the polymeric binder leaving a porous ceramic matrix.The porous ceramic matrix may then be coated by dipping in sols, dryingand sintering to add thin layers to the microporous matrix. These aredifficult and costly methods. It would be advantageous to have analternative method of producing porous ceramic fibres or tubes or hollowceramic membranes.

[0007] Therefore, there is a need in the art for a method of producingporous ceramic fibres or hollow ceramic membranes by electrophoreticdeposition.

SUMMARY OF INVENTION

[0008] The present invention provides methods for producing hollowceramic membranes by electrophoretic deposition. The hollow ceramicmembranes may have a small cross-sectional area of about 1.0×10⁻⁵ mm² toabout 25 mm². The cross-sectional configuration of the hollow ceramicmembranes may be any geometry such as circular, square, rectangular,triangular or polygonal. The hollow ceramic membranes produced by themethods of the present invention may have multiple layers but always theinnermost layer, or the first deposited layer is porous and made byelectrophoretic deposition. Subsequent layers may be porous or nonporous and deposited before or after sintering the first layer. If it isdeposited after sintering, it may require additional sintering steps.Additional layers may be deposited by further electrophoreticdeposition, sol-gel coating, dip coating, vacuum casting, brushing,spraying or other known techniques.

[0009] Therefore, in one aspect of the invention, the invention is amethod of producing a porous hollow ceramic membrane comprising thesteps of:

[0010] (a) providing a suspension of a particulate ceramic material in anon-aqueous liquid;

[0011] (b) electrophoretically depositing the particulate material ontoan electrically conductive fibre core;

[0012] (c) drying the fibre core bearing the deposited material; and

[0013] (d) sintering the fibre core bearing the deposited material at atemperature and for a length of time sufficient to combust the fibrecore while producing a porous hollow ceramic membrane.

[0014] The fibre core may be a bundle of individual fibres which isinfiltrated by the particulate material upon electrophoretic depositionsuch that upon removal of the fibre core, the membrane comprises ahollow core comprising a plurality of elongate cylindrical pores.Alternatively, the fibre core may be coated with an organic binder toprevent infiltration of the particulate material during electrophoreticdeposition.

[0015] In one embodiment, the porosity of the membrane may be controlledby controlling the duration and temperature of the sintering step, bycontrolling the particle size, size distribution and/or the surface areaof the ceramic material, by adding sintering additives in the suspensionwhere the additives will deposit concurrently with the ceramic material,by adding a combustible particulate material, such as carbon, carbonblack or a suitable organic or polymeric material, to the ceramicmaterial which is concurrently deposited with the ceramic material,wherein said combustible material is removed by combustion during thesintering step.

[0016] In one embodiment, the electrophoretic deposition step may berepeated at least once using a ceramic particulate material that isdifferent or has different characteristics such that a multi-layerceramic hollow membrane where each layer has different characteristicsresults. The electrophoretic deposition step may be repeated at leastonce under conditions, as described herein, to produce layers havingdifferent porosities.

[0017] The non-aqueous liquid may be selected from the group comprisingof ethanol, methanol, isopropanol, butanol, acetone, butylamine,acetylacetone methyl ethyl ketone or mixtures thereof.

[0018] In another aspect of the invention, the invention is a method ofproducing a tubular electrode supported electrochemical fuel cellcomprising the sequential steps of:

[0019] (a) electrophoretically depositing an anodic or cathodic materialonto a fibre core to create a porous electrode layer;

[0020] (b) depositing a solid electrolyte layer onto the electrodelayer; and

[0021] (c) drying and sintering the core bearing the deposited anode orcathode layer and the solid electrolyte layer at a temperature and for alength of time sufficient to combust the core and to create a fullydense electrolyte layer while maintaining the porosity of theinner-electrode layer;

[0022] (d) depositing an outer electrode layer onto the solidelectrolyte layer, which is of an anodic material if the inner layercomprises a cathodic material, or a cathodic material if the inner layercomprises an anodic material; and

[0023] (e) sintering the end product at a temperature and for a lengthof time sufficient to bond the outer electrode layer to the solidelectrolyte layer while maintaining the porosity of the outer and innerelectrode layers.

[0024] Preferably, the electrolyte layer is deposited by electrophoreticdeposition.

[0025] In another aspect of the invention, the invention is a method ofproducing a tubular electrode supported electrochemical fuel cellcomprising the sequential steps of:

[0026] (a) electrophoretically depositing an inner electrode layercomprising an anodic or cathodic material onto a fibre core andsintering the core bearing the inner electrode layer at a temperatureand for a length of time sufficient to combust the core and partiallydensify the inner electrode layer while maintaining the porosity of theinner electrode layer;

[0027] (b) depositing a solid electrolyte layer onto the electrodelayer; and

[0028] (c) drying and sintering the core bearing the deposited anode orcathode layer and the solid electrolyte layer at a temperature and for alength of time sufficient to create a fully dense electrolyte layer andbond the electrolyte layer to the inner electrode layer whilemaintaining the porosity of the inner electrode layer;

[0029] (d) depositing an outer electrode layer onto the solidelectrolyte layer, said outer electrode layer comprising an anodicmaterial if the inner layer comprises a cathodic material, or a cathodicmaterial if the inner layer comprises an anodic material; and

[0030] (e) sintering the end product at a temperature and for a lengthof time sufficient to partially densify the outer layer, bond the outerelectrode layer to the solid electrolyte layer while maintaining theporosity of the outer and inner electrode layers.

[0031] Preferably, the electrolyte layer is electrophoreticallydeposited onto the inner electrode layer by inserting an electrophoreticelectrode within the inner electrode layer. Alternatively, the innerelectrode layer is comprised of a cathodic material and is used as theelectrophoretic electrode to electrophoretically deposit the electrodelayer onto the inner electrode layer.

[0032] In yet another aspect of the invention, the invention is a methodof producing a tubular electrode supported electrochemical fuel cellcomprising the sequential steps of:

[0033] (a) providing a porous hollow inner electrode layer comprising ananodic material; (a) electrophoretically depositing a solid electrolytelayer onto the inner electrode layer by inserting an electrophoreticelectrode within the inner electrode layer;

[0034] (b) drying and sintering the core bearing the deposited anode orcathode layer and the solid electrolyte layer at a temperature and for alength of time sufficient to create a fully dense electrolyte layer andbond the electrolyte layer to the inner electrode layer whilemaintaining the porosity of the inner electrode layer;

[0035] (c) depositing an outer electrode layer onto the solidelectrolyte layer, said outer electrode layer comprising a cathodicmaterial; and

[0036] (d) sintering the end product at a temperature and for a lengthof time sufficient to partially densify the outer layer, bond the outerelectrode layer to the solid electrolyte layer while maintaining theporosity of the outer and inner electrode layers.

BRIEF DESCRIPTION OF DRAWINGS

[0037] The invention will now be described by way of an exemplaryembodiment with reference to the accompanying drawings. In the drawings:

[0038]FIG. 1 is a schematic of an EPD setup showing particle depositionon a fibre electrode.

[0039]FIG. 2 is a graphical representation of a sintering cycle of thepresent invention.

[0040]FIG. 3A is a scanning electronic micrograph (SEM) of a fracturesurface hollow ceramic membrane at 70× magnification. FIG. 3B is thesame fraction surface at a higher magnification.

[0041]FIG. 4A is a SEM of a cross-sectional fracture surface of a hollowceramic membrane sintered at 1550° C. for 5 hours. FIG. 4B is a SEM of asimilar membrane sintered at 1400° C. for 5 hours. FIG. 4C is a SEM of asimilar membrane sintered at 1250° C. for 5 hours. Each SEM is at 2000×magnification.

[0042]FIG. 4D is the same fracture surface as FIG. 4C at a highermagnification.

[0043]FIG. 5 is a SEM of a polished cross-sectional surface showingporosity created by carbon black particles in the deposition suspension(2000×).

[0044]FIG. 6A is a SEM of a cross-sectional fracture surface showing aporous core resulting from the use of untreated fibre tow as the fibrecore (70×). FIGS. 6B and 6C is the same fracture surface at highermagnifications (300× and 650×).

[0045]FIG. 7 is a SEM of a cross-sectional fracture surface of a ceramichollow membrane produced using a fibre core comprising fibre tow havinga diameter of approximately 400 microns.

[0046]FIG. 8 is a SEM of a cross-sectional fracture surface of a ceramichollow membrane produced using a fibre core comprising fibre tow havinga diameter of approximately 650 microns.

DETAILED DESCRIPTION

[0047] When describing the present invention, the following terms havethe following meanings, unless indicated otherwise. All terms notdefined herein have their common art-recognized meanings.

[0048] The term “fibre” or “filament” refers to a single strand offibrous material; “fibre tow” or “fibre bundle” shall refer to amulti-filament yarn or an array of fibres; and “fibre core” shall referto a fibre, filament, fibre tow or fibre bundle. In all cases, the fibrecore is electrically conductive or treated to be electrically conductiveto allow its use as an electrode.

[0049] The term “ceramic” refers to inorganic non-metallic solidmaterials with a prevalent covalent or ionic bond including, but notlimited to metallic oxides (such as oxides of aluminium, silicon,magnesium, zirconium, titanium, chromium, lanthanum, hafnium, yttriumand mixtures thereof) and nonoxide compounds including but not limitedto carbides (such as of titanium, tungsten, boron, silicon), silicides(such as molybdenum disicilicide), nitrides (such as of boron,aluminium, titanium, silicon) and borides (such as of tungsten,titanium, uranium) and mixtures thereof; spinels, titanates (such asbarium titanate, lead titanate, lead zirconium titanates, strontiumtitanate, iron titanate), ceramic super conductors, zeolites, ceramicsolid ionic conductors (such as yittria stabilized zirconia,beta-alumina and cerates).

[0050] The term “hollow ceramic membrane” shall refer to a small(1.0×10⁻⁵ mm²-25 mm²) cross-sectional area ceramic body comprising atleast one layer of a porous ceramic material. In a multilayer membrane,the innermost layer is porous and the subsequent layers may be porous ornonporous. The cross-sectional geometry of the hollow ceramic membranesmay be any shape such as circular, square, rectangular, triangular orpolygonal.

[0051] The term “porous”, in the context of hollow ceramic membranesmeans that the ceramic material contains pores (voids). Therefore, thedensity of the porous ceramic material is lower than that of thetheoretical density of the ceramic material. The voids in the porousceramics can be connected (i.e., channel type) or disconnected (i.e.,isolated). In a porous hollow ceramic membrane, the majority of thepores are connected. To be considered porous as used herein, a ceramicmembrane should have a density which is at most about 95% of thetheoretical density of the material. The amount of porosity can bedetermined by measuring the bulk density of the porous body and from thetheoretical density of the materials in the porous body. Pore size andits distribution in a porous body can be measured by mercury ornon-mercury porosimeters, BET or microstructural image analysis as iswell known in the art.

[0052] The present invention provides an electrophoretic method ofproducing hollow ceramic membranes. The methods disclosed herein may beused to produce such membranes having multiple concentric layers ofvarying compositions. One particular application of such methodsincludes the production of solid oxide fuel cell tubes. As well, themethods disclosed herein may be used to produce functionally gradedhollow ceramic membranes where either material composition, amount ofporosity, pore size distribution or the microstructure, or a combinationof these characteristics, may vary along the cross section.

[0053] EPD (electrophoretic deposition) is an electrochemical depositiontechnique for depositing minute particles of materials such as metals,glass, ceramics, polymers or carbon in a colloid suspension bysubjecting the particles to an external dc electric field, therebycausing the migration of the particles toward a specific electrode.Particles in a colloid are known to develop a surface charge relative tothe suspension medium, which may be dependent on the pH of thesuspension medium. For example, alumina has a positive charge as aresult of ionization at a pH of below about 7. In the formation ofceramic green bodies by EPD, the ceramic particles may be positively ornegatively charged; in case of positively charged particles they aredeposited on the cathode; and in case of negatively charged particlesthey are deposited on the anode. It is not essential for the depositionprocess that the particles have to reach the oppositely chargedelectrode; particles can be deposited around an electrode onto asemipermeable membrane which allow ions to pass but not the particlesthemselves. The oppositely charged electrode on which the ceramicparticles are deposited (in the absence of a semipermeable membrane) isreferred to herein as the “deposition electrode”. This is shownschematically in FIG. 1.

[0054] The ceramic material used in the present invention may includethose compounds referred to above or mixtures of thereof. However, theinvention is not limited to any chemical compound specifically referredto herein and should be considered to include any ceramic or similarmaterial which may be electrophoretically deposited from a non-aqueoussuspension in accordance with the methods disclosed herein.

[0055] The present invention may be utilized to electrophoreticallydeposit a plurality of coatings on a wide range of fibre cores, bothmetallic and non-metallic. Any fibre core with any small cross-sectionalgeometry may be coated with the disclosed methods if it is electricallyconductive or may be treated to be electrically conductive and may becombusted at temperature levels reached during a sintering process.Fibre cores made from carbon or graphite are considered most suitablefor use herein.

[0056] The present invention may be used with fibre cores of varyingdiameters. At one end of the range, individual filaments having adiameter of approximately 5 microns or less may be suitable to producevery fine hollow ceramic membranes. At the other end of the range, fibretow having a diameter of about 5 or 6 mm may be used to produce largerhollow ceramic membranes. At the larger end of the range, rods having adesired diameter may be used in place of fibre tow. As well, the rodsmay have any suitable cross-sectional configuration.

[0057] Fibre tow may be used either treated with a polymeric binder oruntreated. A treated fibre core will produce a ceramic tube havingsubstantially a single hole as seen in FIGS. 3A and 3B. A fibre coremade from untreated fibre tow may result in a ceramic tube having aplurality of holes in a porous core, as is seen in FIGS. 6A, 6B and 6C.The fibre tow may be treated by briefly dipping the tow into a solutionof an organic or polymeric binder before immersion in theelectrophoretic medium. In one example, a solution of nitrocellulose inacetone is suitable. The nitrocellulose forms a very thin coating on thetow and seals the interfilamentous gaps. The binder should preferably beinsoluble in the EPD medium. Nitrocellulose is a preferred binderbecause it is insoluble in ethanol, which is a preferred EPD medium.

[0058] Fibre tow which has been treated with an organic binder may befashioned into a shaped deposition electrode, other than a simpleelongate deposition electrode, by manipulating the fibre tow before thebinder dries. For example, the fibre tow may be fashioned into adeposition electrode having helical shape or a “U” or “J” shape. Theresulting hollow ceramic membrane will of course have the shape of thedeposition electrode, which may be useful in certain applications.

[0059] If the intrafilamentous gaps are unsealed, as in untreated fibretow, the deposited particles may infiltrate the tow during thedeposition process, resulting in the porous core referred to above. Theporous core may be preferred in some applications in which a highinternal surface area may be beneficial. Examples of such applicationinclude high surface area catalyst supports or membrane reactors.

[0060] The ceramic material used in the deposition process preferablycomprises particulate ceramic material having a particle size largerthan about 150 nm. A suitable suspension of the ceramic material may bemade by grinding ceramic powder using grinding media in a suitablenon-aqueous medium i.e. an organic liquid, such as ethanol, isopropanol,butanol, butylamine, acetylacetone, methyl ethyl ketone, acetone,methanol, absolute alcohol or mixtures thereof for a specified period oftime until the average particle size reaches the appropriate size range.In one embodiment, the particle size may range from about 150 nm toabout 10,000 nm. The particles should preferably be no larger than about15,000 nm. More preferably, the particle size range may be between about200 nm to about 1000 nm. As will be appreciated by those skilled in theart, larger particle sizes may result in a ceramic membrane havinggreater porosity than a ceramic membrane resulting from smaller particlesizes.

[0061] As shown schemicatially in FIG. 1, the EPD process may commenceby providing a suitable length of fibre core and connecting it to asuitable EPD apparatus which is well known in the art. The fibre coremay then be immersed in the ceramic particle suspension and electricpotential applied at a specified level for a suitable length of time. Itmay be important to pre-immerse and withdraw the fibre core once beforeimmersion for EPD in the EPD suspension or any liquid. During earlytrials, it was found that if such a step was not performed, theresulting coated fibre core often demonstrated gross irregularities indiameter after deposition. This problem was alleviated by immersing thefibre tow into the suspension and slowly removing it. It is believedthat during removal, surface tension forces pulled all the individualfilaments of the tow together to ensure the tow has a uniform rounddiameter when re-immersed for EPD. This step is unnecessary for singlefilament fibre cores or rods.

[0062] The choice of appropriate EPD conditions such as current, voltageand length of time will vary with the desired end product and is wellwithin the ordinary skill of one skilled in the art. In one embodiment,the current may vary from about 0.01 mA to about 1.0 mA per centimeterof deposit length, over a time period of about 30 seconds to about 300seconds. The current and length of deposition may be varied to achievemembranes of differing thickness. After EPD, the coated fibre core isthen removed from the suspension and dried in preparation for sintering.The drying step may take place at room temperature or a slightlyelevated temperature.

[0063] In one embodiment, the sintering cycle for an alumina or zirconiadeposit where the sintering atmosphere is air may begin by raising thetemperature to about 500° C. to about 900° C. over a period of about 6hours to about 9 hours and held at that temperature for about 3 hours.The temperature may then be raised at a rate of about 100° C. to about500° C. per hour to the sintering temperature of about 1150° C. to about1500° C. and held there for about 0.5 to about 10 hours. The temperaturemay then be lowered at a rate of about 100° C. to about 500° C. per hourto room temperature. One example of a sintering cycle falling withinthis embodiment is illustrated graphically in FIG. 2.

[0064] In another embodiment, the sintering cycle for a boron carbidedeposit where the sintering atmosphere is a vacuum or argon may begin byraising the temperature to about 600° C. over a period of about 3 hoursto about 6 hours and held at that temperature for about 1 hour. Thetemperature may then be raised to about 900° C. to about 1100° C. over0.5 to about 5 hours and held there for about 0.5 to about 3 hours. Thetemperature may then be raised by about 300° C. to about 800° C. perhour up to the sintering temperature of about 1800° C. to about 2250° C.and held there for about 0.25 to about 5 hours. The temperature may thenbe lowered at a rate of about 100° C. to about 800° C. per hour to roomtemperature.

[0065] It may be important to hang the ceramic tubes vertically duringsintering to prevent curvature of the tubes. Curvature of the tubes maybe the result of nonisotropic heat transfer kinetics if the ceramictubes are laid flat on supports during sintering or due to the frictionbetween the support and the hollow ceramic membrane.

[0066] Porosity of the ceramic tubes or hollow fibres or hollow ceramicmembranes may be enhanced by mixing combustible particles such as carbonblack, carbon, graphite, different polymer powders and cellulose basepowders into the ceramic particle suspension such that the combustibleparticles co-deposit during EPD. Then, when the ceramic coated core isheated to sinter the hollow ceramic membranes and remove the core, thecombustible particles will also be burned off, resulting in a moreporous ceramic membrane.

[0067] As well, porosity may be controlled by controlling thetemperature and time of the sintering process. Long sintering times orsintering at higher temperature or combination of both can reduceporosity. Porosity can also be controlled by controlling the particlesize distribution and its surface area. Finer and high surface areaceramic particles normally will have lower porosity than coarse and lowsurface area powder when both of them are sintered under identicalconditions. Porosity can also be controlled by sintering additives whichare well known in the art, such as glassy or sol-gel phase or any otherliquid forming phases. The time and temperature parameters in a typicalsintering cycle, such as that illustrated in FIG. 2 may be varied by oneskilled in the art to achieve a particular desired result.

[0068] A functionally graded composite hollow ceramic membrane may beproduced by varying the composition of the EPD suspension during thedeposition process. For example, a suspension of yttria stabilizedzirconia may be continuously added to a suspension of alumina during EPDto produce a ceramic tube which has a gradient of YSZ compositionranging from none or very little near the inner diameter of the membraneto close to 100% near the outer diameter of the membrane. Alternatively,the fibre core may be deposited onto from a series of suspensions havingvarying proportions of the materials desired in the composite endproduct. Similarly, a porosity or pore size distribution graded hollowceramic membrane can be fabricated by changing the powder compositionsuch a way that the sinterability of the deposit varies along the crosssection. In one embodiment, the particle size distribution may changedduring the deposition step, which may result in a porosity gradedmembrane. Also, porosity graded hollow ceramic membranes can bemanufactured by changing the concentration or particle size of acombustible material such as carbon black, carbon, graphite or polymerpowders along the cross-section of the deposit. Porosity may also bevaried along the cross-section of the membrane by varying theconcentration of a sintering aid during the deposition step.

[0069] A thin surface layer of a functional material may be added to thehollow ceramic membrane by sol-gel coating, dip coating, electrolessmetal coating, polymer coating, vacuum casting, spraying or brushing orother well-known techniques for coating tubes or hollow fibres. Theresulting coated hollow ceramic membrane may be used as a solid oxidefuel cell, a separation membrane or in a membrane reactor, amongst otheruses. The hollow ceramic membranes will act as a support or substratefor the functional surface layer which will have the desired propertiesfor a particular application. For example, a separation membranerequires a specific pore size or size distribution. The support layerwill have a larger pore size than the final layer but the surface layerwill generally be thinner. So the function of the support layer in thisparticular example to provide mechanical support to the final layerbecause a self supporting thin functional layer can not be made. In oneparticular example, a thin palladium layer may be added to a hollowceramic membrane by electroless plating. The palladium/ceramic tube maybe used in a membrane separator for separating hydrogen which ispermeable through palladium, which is otherwise non-permeable.

[0070] In one embodiment, multiple concentric layers may be depositedusing the methods of the present invention. If the first or innermostlayer is porous, it may act as a semipermeable membrane and allowfurther EPD to deposit additional layers. A series of deposition stepswith different ceramic materials may be followed by a single sinteringstep. Alternatively, a sintered hollow ceramic membrane may be used todeposit an additional layer or layers of ceramic material by furtherEPD, sol-gel coating, dip-coating, vacuum casting, spray coating orsimilar technologies which is then sintered. The different layers maychosen to provide the finished product with a variety of differentproperties or capabilities for a variety of applications.

[0071] In one embodiment of a multilayer ceramic tube, the methodsdisclosed herein may be used to produce a tubular solid oxide fuel cell.In a typical planar electrode supported SOFC, the cell is comprised of aporous anode layer, a fully dense electrolyte and a porous cathodelayer. Electrochemical reactions in the anode and cathode produceelectricity. The electrolyte permits flow of oxygen ions from thecathode to the anode where a fuel is oxidized to release electrons. Theelectrons travel back to the cathode externally to complete the circuit.The fuel cell may be anode-supported in which case the anode layer willbe thicker than the electrolyte of cathode layers. In one example, theanode layer may typically be about 0.1 to about 3 mm thick, preferablyabout 1.5 mm thick, and comprise a nickel/yttria stabilized zirconia(“YSZ”) composite material. The electrolyte may be fully dense YSZ about0.002 to about 0.1 mm thick, preferably about 0.01 mm thick, while thecathode may be comprised of a mixed conductor oxide layer such as LSMwhich may be about 0.01 mm to about 0.5 mm thick, preferably about 0.04mm thick. Such planar solid oxide fuel cells are well known in the art.

[0072] To produce a tubular SOFC in one embodiment, an inner electrodelayer, which may be either an anode or a cathode, is deposited onto thefibre core to a desired thickness. Secondly, a thin solid electrolytelayer is then deposited. This intermediate product is then sintered toachieve full density of the solid electrolyte layer while maintainingthe porosity on the inner electrode. By using different average particlesizes for the solid electrolyte and anode deposition suspensions, theanode layer and the electrolyte layer will have two different sinteringkinetics so that for a given sintering cycle, the electrolyte would benonporous (fully dense) but the anode would be porous. The porosity ofthe anode layer may be enhanced using combustible particles into theanode particle suspension. In addition or alternatively, sinteringadditives may be added to the electrolyte layer to enhance thedensification of the electrolyte layer. Lastly, the cathode layer isdeposited onto the electrolyte layer by any suitable means includingdip-coating, brushing, spraying or sol-gel coating, followed by a finalsintering stage of the tubular SOFC membrane to partially densify theouter cathode layer and bond the outer cathode layer to the electrolytelayer.

[0073] It is possible to electrophoretically deposit the outer cathodelayer onto the electrolyte layer if a thin layer of a conductingmaterial is applied to the electrolyte layer so that it may be used asan deposition electrode.

[0074] Various alternatives to this method are possible. In oneembodiment, the inner layer is a cathodic material which will remainelectrically conductive after sintering. In this case, the electolytelayer may be electrophoretically deposited onto the cathode layer evenafter the fibre core has been removed when sintering the cathodic layer.

[0075] In another embodiment, the inner layer is an anodic materialwhich may not be conductive. In this case, the electrolyte layer may beelectrophoretically deposited onto the anode layer, after the anodelayer has been sintered and the fibre core removed, by inserting adeposition electrode within the hollow anode layer and filling thehollow anode layer with the EPD medium such as ethanol, without theparticulate suspension. Because of the porosity of the anode layer, theelectric field will penetrate the anode layer, causing the electrolyteparticle to deposit onto the anode layer. In other words, the anodelayer is acting as a semi-permeable membrane during EPD. After theelectrolyte layer is deposited, the anode and electrolyte layers aresintered to densify the electrolyte layer while maintaining the porosityof the anode layer. The cathodic layer may then be added by any suitableconventional technique such as dip coating or sol-gel coating.

[0076] This alternative method may be applied to any hollow ceramicmembrane whether or not it has been formed by EPD. For example, a hollowceramic membrane may be formed by extrusion or slip casting which iselectrically nonconductive. The membrane may then be immersed in an EPDsuspension and filled with the EPD medium. A deposition electrode maythen be inserted within the hollow membrane and another layer of ceramicmaterial deposited onto the hollow membrane. The hollow membrane must ofcourse be sealed at the end which is immersed into the EPD suspension.

[0077] The following examples are intended to illustrate specificembodiments of the present invention and should not be consideredlimiting of the claimed invention in any way.

EXAMPLE 1

[0078] 100 g of alumina powder was mixed with 250 g of YSZ grindingmedia (5 mm diameter) in 200 ml of absolute ethanol and vibromilled forabout 15 hours. The resulting alumina particle size ranged from 200 to2000 nm, with a median diameter of about 300 nm. Approximately 750 ml ofethanol was added to dilute the alumina to about 10 g per 100 ml ofethanol. The suspension was acidified using concentrated acetic acid ordilute HCl (or both) to a pH of below 5, preferably about 4. Thesuspension was then tested for appropriate deposition.

[0079] A carbon fibre tow of about 100 micron diameter was prepared bydipping briefly in a nitrocellulose/acetone solution. The suspension wasdeposited onto the fibre core with a current of about 0.5 mA for about300 seconds. The resulting coated fibre core was air dried for about 12hours and then sintered. The sintering cycle began by raising thetemperature to about 900° C. over a 9 hour period and held there forabout three hours. The temperature was then raised by 300° C. per hourup to the sintering temperature of 1400° C. and held there for about 5hours. The ceramic tube was then allowed to cool at a rate of 300° C.per hour to room temperature. A cross-section of the resulting poroustube is seen in FIGS. 3A and 3B.

EXAMPLE 2

[0080] In one example, particulate carbon black having a particle sizerange of about 20 nm to about 150 nm, was added to the aluminasuspension prepared in accordance with Example 1 above in about 29 v/osuch that the carbon and alumina were co-deposited onto the fibre core.The resulting ceramic membrane was sintered at 1250° C. for 5 hours.FIG. 4 shows that the carbon black increases the porosity of themembrane.

EXAMPLE 3

[0081] An untreated fibre tow was used to deposit alumina particles asotherwise described in Example 1 above. The resulting ceramic membranehaving a porous core is shown in FIGS. 5A and 5B. The core has aplurality of longitudinal holes, which correspond to individualfilaments of the fibre tow.

EXAMPLE 4

[0082] Fibre cores comprising fibre tow of varying sizes was used fordepositing alumina particles as otherwise described in Example 1 above.The fibre tow diameters used were approximately 400 μm (FIG. 7) to 650μm (FIG. 8). The resulting ceramic tubes are shown in FIGS. 7 and 8.

[0083] As will be apparent to those skilled in the art, variousmodifications, adaptations and variations of the foregoing specificdisclosure can be made without departing from the scope of the inventionclaimed herein.

1. A method of producing a tubular electrode supported electrochemicalfuel cell comprising the sequential steps of: (a) electrophoreticallydepositing an anodic or cathodic material onto a fibre core to create aporous electrode layer; (b) depositing a solid electrolyte layer ontothe electrode layer; and (c) drying and sintering the core bearing thedeposited anode or cathode layer and the solid electrolyte layer at atemperature and for a length of time sufficient to combust the core andto create a fully dense electrolyte layer while maintaining the porosityof the inner electrode layer; (d) depositing an outer electrode layeronto the solid electrolyte layer, which is of an anodic material if theinner layer comprises a cathodic material, or a cathodic material if theinner layer comprises an anodic material; and (e) sintering the endproduct at a temperature and for a length of time sufficient to bond theouter electrode layer to the solid electrolyte layer while maintainingthe porosity of the outer and inner electrode layers.
 2. The method ofclaim 1 wherein the electrolyte layer is deposited by electrophoreticdeposition.
 3. A method of producing a tubular electrode supportedelectrochemical fuel cell comprising the sequential steps of: (a)electrophoretically depositing an inner electrode layer comprising ananodic or cathodic material onto a fibre core and sintering the corebearing the inner electrode layer at a temperature and for a length oftime sufficient to combust the core and partially densify the innerelectrode layer while maintaining the porosity of the inner electrodelayer; (b) depositing a solid electrolyte layer onto the electrodelayer; and (c) drying and sintering the core bearing the deposited anodeor cathode layer and the solid electrolyte layer at a temperature andfor a length of time sufficient to create a fully dense electrolytelayer and bond the electrolyte layer to the inner electrode layer whilemaintaining the porosity of the inner electrode layer; and (d)depositing an outer electrode layer onto the solid electrolyte layer,said outer electrode layer comprising an anodic material if the innerlayer comprises a cathodic material, or a cathodic material if the innerlayer comprises an anodic material; and (e) sintering the end product ata temperature and for a length of time sufficient to partially densifythe outer layer, bond the outer electrode layer to the solid electrolytelayer while maintaining the porosity of the outer and inner electrodelayers.
 4. The method of claim 3 wherein the electrolyte layer iselectrophoretically deposited onto the inner electrode layer byinserting an electrophoretic electrode within the inner electrode layer.5. The method of claim 3 wherein the inner electrode layer is comprisedof a cathodic material and is used as the electrophoretic electrode toelectrophoretically deposit the electrode layer onto the inner electrodelayer.
 6. A method of producing a tubular electrode supportedelectrochemical fuel cell comprising the sequential steps of: (a)providing a porous hollow inner electrode layer comprising an anodicmaterial; (b) electrophoretically depositing a solid electrolyte layeronto the inner electrode layer by inserting an electrophoretic electrodewithin the inner electrode layer; (c) drying and sintering the corebearing the deposited anode or cathode layer and the solid electrolytelayer at a temperature and for a length of time sufficient to create afully dense electrolyte layer and bond the electrolyte layer to theinner electrode layer while maintaining the porosity of the innerelectrode layer; and (d) depositing an outer electrode layer onto thesolid electrolyte layer, said outer electrode layer comprising acathodic material; and (e) sintering the end product at a temperatureand for a length of time sufficient to partially densify the outerlayer, bond the outer electrode layer to the solid electrolyte layerwhile maintaining the porosity of the outer and inner electrode layers.